Bleaching pulp without direct chlorination by bleaching with chlorine dioxide



FIFSZO? XR BLEACHING PULP WITHOUT DIRECT CHLO- RINATION BY BLEACHING WITH CHLO- RINE DIOXIDE Amiel W. Brinkley, Jr., Mobile, Ala., and Jack F. Kilborn, Natchez, Miss, assignors to International Paper Company, New York, N.Y., a corporation of New ork No Drawing. Continuation of application Ser. No. 285,578, June 5, 1963. This application June 30, 1966, Ser. No. 562,963

3 Claims. (Cl. 162-78) The process of this invention is particularly suitable for use upon dissolving grades of cellulosic pulp pre pared or post-treated in accordance with co-pending US. application, Ser. No. 176,489, filed Feb. 23, 1962 as a continuation-in-part of US. application 810,877, filed May 4, 1959 and both now abandoned. Indeed, the present application must properly be regarded as a continuation-in-part of Ser. No. 176,489, filed Feb. 23, 1962, and is a continuation of Ser. No. 285,578 filed June 5, 1963, now abandoned.

This invention relates to an improvement in the process of manufacturing a dissolving grade of cellulosic pulp for use in the manufacture of textile or tire grade rayon, cellophane or other cellulosic products manufactured by the viscose process. More specifically, this improvement involves a new method of bleaching dissolving grades of cellulosic pulps which improves the viscose filterability and the rayon strength properties of these pulps. This improved bleaching sequence involves omission of the usual direct chlorination stage, but produces pulps with high brightness and high viscosity by a combination of hypochlorite, caustic extraction and chlorine dioxide stages.

Historically, direct chlorination, which is treatment of the pulp with elemental chlorine gas, has been necessary as a preliminary bleaching stage in order to produce pulps with high brightness without undue degradation of the cellulose. This direct chlorinaiton reacts with the lignin in the unbleached pulp rendering it soluble in caustic and therefore, the chlorination stage is generally followed by a caustic extraction stage to dissolve the chlorinated lignins. Thus, for example, where the lignin content of unbleached dissolving pulps made from softwoods such as southern pine is approximately 3 to 4% when the pre hydrolyzed kraft cooking process is used and where dissolving pulps cooked in the same manner from hardwoods such as sweetgum or oak, as well as grasses such as bamboo or sugar cane bagasse, have a lignin content of about 1 to 2% before bleaching, the combination of chlorination and caustic extraction normally removes approximately 80% of the lignin present in the unbleached pulp. Following the chlorination and caustic extraction stages, additional bleaching stages are necessary to remove the residual lignin and coloring matter in the pulp to produce high brightness and purity.

As recently as 1953, the bleaching chemicals available to complete the bleaching and purification of pulps degraded the cellulose rather severely at the same time that these chemicals were removing the lignin and coloring matter from the pulp. Hypochlorite or chlorine dioxide under alkaline conditions were generally used. However, in recent years, the use of chlorine dioxide under 3,345,250 Patented Oct. 3, 1967 acid conditions has developed to the point where cellulosic fibers can be bleached to high brightness and purity with very little degradation of the cellulose. The development of corrosion resistant materials and processes for economically generating chlorine dioxide have made the use of this chemical in large quantities possible and economically practical.

It has now been found that cellulosic materials such as pulps from hardwoods or grasses, which have a fairly low lignin content before bleaching, can be bleached fairly economically by a sequence which omits the chlorination stage and instead uses chlorine dioxide or a mild hypochlorite treatment as the first bleaching stage. Not only is such a sequence economically practical, but it also produces pulps which have better viscose filterability and rayon strength stability than pulps bleached by a normal sequence which includes a chlorination stage. This unexpected phenomenon has been proved repeatedly in the laboratory and in mill trials.

It has now also been found that, when the chlorination stage is omitted, it becomes necessary to treat the pulp in the first stage with some type of acid in order to reduce the minerals which otherwise interfere with the bleaching and result in a low pulp brightness. Presumably, the acid conditions that exist in a normal chlorination stage are sufiicient to remove these minerals which are primarily iron and manganese, and therefore no acid treatment is required for this type bleaching.

The fact that softwood kraft dissolving pulps have almost twice as much lignin or bleaching chemical demand as similar pulps from hardwoods or grasses and the fact that chlorine dioxide costs twice as much as the equivalent amount of chlorine result in a large cost penalty for bleaching a softwood kraft pulp without chlorination. Nevertheless, softwood pulps can be bleached in this manner with a view to achieving improved viscose filterability and rayon strength stability compared to similar pulps bleached with the normal sequence involving a chlorination stage.

Again dissolving pulps cooked by other processes such as the sulfite process and which have relatively low lignin contents before bleaching can be bleached readily without a chlorination stage to produce rayon with better viscose filterability and rayon strength properties than a similar pulp bleached with chlorination.

For a clearer understanding of the present invention, reference should be made to attached Tables 1 to 4 which summarize the work done to provide the requisite showing to enable any person skilled in the art to use such invention and to provide a means of comparing the results of such work with the results of other work based on the utilization of chlorine gas as a bleaching agent. Table 2 shows, for instance, that textile grade rayon from bamboo pulp bleached without chlorination had about the same airdry tensile strength, but significantly better tensile strength after ovendrying at C. for 16 hours than did rayon from the same kind of pulp, which was bleached with chlorination. Viscose filterability was also improved by bleaching without chlorination. Again, Tables 3 and 4 show that, within their ambit, bleaching without chlorination significantly improved the airdried tensile strength and elongation as well as the wet tensile strength and elongation.

TABLE 1.--BAMBOO DISSOLVING PULP TABLE 2.BAMBOO DISSOLVING PULP Raw material Bamboo Raw material Bamboo Type bleaching Without With Chlorination Chlorination without w Type bleaching Chlprin- Chlorin- Permanganate number 8.4 8.4 at on 8mm Uiifleaghfid viscosity, TAPPI T230, 130 130 D Com osite number -224 00-22 Bleaching sequence. AHDEDH X CEHEDH l Bleaiflng sequence AHDED 1 CEHED3 1 Composite number 224 223 I. Chlorination or Sulfuric Acid (3.0% Viscose Solution Quality consistency, 60 minutes):

ghgerceutni "636. 2. 45 cen Tgm ergsiire, s2 s1 Aging t mperature, 0-- 25. 0 25.0 Resi ual, g.p.l. available C11 0. 10 Aging time, hr. (40 Visa). 25 25 Il': Caustic Extraction or Sodium Hy. Temperature reguiremen 23. 0 23. 2 polslhlgrge (120 m mutes retention) 1 39 1 25 gr l r r igr v iysco sgtg ifi 155$ 35! 35 a ercen NaO Clgerce 3. 72 Fil rability, nylon, percent 75 Plug Temperature, 111 160 Filtelability. 17 P ing volume. 86 34 Consistency, per 12. 0 10. 0 11 off 10. 3 10. 3

esidual, av 0. 20 Residual NaOH, g.p.l 0. 37 Rayon Quality Viscosity, TAPPI Denier, g 173. 0 174. 6 1 10 Airdry: Tensile, g./denier cond 2.41 2.41 '55 Elongation, percent cond. 17. 7 18. 0 g Tensile, gJdenier wet 1.09 1. 07 1o 0 12 0 Elongation, percent wet 34. 5 34. 6

5 Ovendry reconditioned: 4, 2 m 2 e-l g a 2 3; 0- 9; esidual, g.p.l. available 011 0.10 0.14 igfi: Peicen 4 g./deu1er wet 1. 03 0. 22 Viscosity,3p TAPPI T 230, 1% 27 5 27 30 H iillongation 30 wet ea oss, percen gg g gggfiag gig Rayon brightness, desuliured 71. 6 69. 6 Naofi percent: o 50 25 Rayon color, desulfured 38. 2 43. 0 pH 05 11. 2 11.4 fitiesidugl NaICA 121g, PgipJ i 2 3 0 0. 33 0. 21

scos1 y CED, '38 26 5 2&0 35 Physical Properties V. Chlorine loxide (10.0% consistene p 0 25 Basis weight, 1b./1,0o0 sq. it 143. 4 143. 4 p r 0 160 Apparent density, g./cc 0. 72 0. 72 Retentwn, mmutes- 80 80 Brightness, percent G.E. 91.1 91.0 87351.11"gar-tenement" 0 1% 0 13 4 n t 's fiiz iif 553 353 1r 0 s s Viscosity: TAPPI 'r-230, 1% p q CED, cp 26.2 24.8 Sodium Hypochlorite (one or more stages at 6.5% consistency for 3 hours Chemical Analysgs at temperature and chemical concentraitziogi necessary for desired v scos Final GE. brightness, TAPPL 91.1 91. o Pulp TAPPI T430. 1 16-8 17. 8 12 15 12 33" 16 8 8 tti filit fi titfit 222 2'? Total Chemicals (e'er'itea'ib'r'i'e' Gamma cellulose, percent- 1.3 1.3 0111811115 and sodium hypochlonte gg gg g g -3 3 r cl 241mm 4, 05 3 3 Resins, percent.. 0. 082 0. 106

2. NaO ,percen 7.21 3.40 e, p-p.m-.... 20 22 0101, percent. 0. 82 0. 25 P-P- 46 23 i" 33 3'55 1%; 3'1 3'3 ercen p 7.14% NaOH solubility 10.6 10.4 1 A-suliuric acid pretreatment, C-chlorination, E-caustic extrac- Copper number 68 tion, D-chlorine dioxide, H-sodium hypochlorite.

'- After the hypochlorite stages, these pnlps were centri-cleaned and 55 given two sulfur dioxide treatments for 30 minutes each, 5.0% consistency, .6 to 3.0 pH, and 125 to 130 F. before making viscose handsheets.

TABLE 3.-HARDWOOD DISSOLVING PULPS-BLEACHING A-sulfuric acid pretreatment, C-chlorination, E-caustic extraction, D-chlorine dioxide, H-sodium hypochlorite.

Bleaching sequence CDEHDED 1 A EHDED 1 Type pulp Mixed Sweetgum Mixed Sweetgum Hardwoods Hardwoods Pennanganate number 5.6 4.3 5.6 4.3

Viscosity, TAPPI T-230, 1% CED, cp 187 158 187 158 Bleach composite number 245 248 247 246 I. Sulfuric Acid or Chlorination (3.0% consistency,

60 minutes):

011, percent 1.30 0.75 C101, percent 0. 10 0. 10 0. 40 0. 40 H1804, percent 0.85 0.79 pH adjusted to 2. 8 2. 8 Temperature, F 79 100 pH 011 2. 7 2. 7 Residual, 30 min/60 min., g.p.l. available 012.... 0. 09/0. 06 0. 03/0. 01 0. 03/0. 01 0. 02/0. 01

TABLE 3Con-tinued Bleaching mmn IC EHDED 1 A,,EHDED 1 Type pulp Mixed Sweetgum Mixed Sweetgum Hardwoods Hardwoods Permanganate number 5.6 4.3 5.6 4.3 Viscosity, TAPPI T-230, 1% CED, cp 187 158 187 158 Bleach composite number 245 248 247 246 II. Caustic Extraction (10.0% consistency, 120

minutes):

NaOH, percent 0.54 0. 40 0.42 0. 32 Temperature, F- 160 130 160 130 pH on 11.4 11.2 11.4 11.3 pH off 9.8 10.2 10. 6 10.4 Residual, g.p.l. NaOH 0. 17 0.08 0. 18 0. 09 Viscosity, TAPPI T-230, 1% CED, cp 141 122 163 140 III. Sodium Hypochlorite (10.0% consistency, 120

minutes):

NaO Cl, percent available Oh 0.30 NaOH, percent. 0. 09 Temperature, F- 90 pH oft l0. 6 Residual, g.p 1e 0. 12 G.E. brightness, heatdry 72 Viscosity, TAPPI T230, 1% D, cp 149 IV. Chlorine Dioxide (10.0% consistency, 180 minutes, 140 F.):

0102, percent 0. 42 0.49 NaOH, percent- 0. 08 H 011 3.0 3. lesidual, g.p.l. av ble Clz 0.18 0.13 NaOH for neutralization, percent 0.15 0.15 pH after neutralization 7. 6 8.0 G.E. brightness, heatdry 83 82 Viscosity, TAPPI T-230, 1% CED, cp 131 124 V. Caustic Extraction (10.0% consistency, 120 minutes, 140 F.):

NaOH, percent 0.22 0.18 0. 0.22 pH oti 10.9 10.8 11. 0 10.9 Residual, g.p.l. N 0.16 0.14 0. 16 0.16 Viscosity, TAPPI T-230, 1% CED, cp 108 121 116 VI. Chlorine Dioxide (10.0% consistency, 180 minutes, 140 F.):

0102, percent 0. 20 0. 20 0.21 0. 20 p o 4. 4 4.3 4.3 4. 2 Residual, g.p.l. avail 0.19 0.15 0.18 0.16 NaOH for neutralization, percent 0. 06 0.05 0.06 0.05 pH after neutralization 8. 4 8. 2 8. 0 8. 1 G.E. brightness, heat 89 87 88 88 Viscosity, TAPPI T-230, 1% CED, cp 111 103 116 112 VII. Sodium Hypochlorite (one or more stages at 6.5% consistency for 3 hours at temperatures and chemical concentrations necessary for desired viscosity):

Final G.E. brightness, TAPPI 2 89.4 88.4 88.7 87.4 Final Viscosity, TAPPI T-230, 1% CED, op--- 100 100 101 102 Total Chemicals (corrected for excess chlorine and sodium hypochlorite residual):

C12, percent 1.42 0.75 0.23 N aOH, percent. 0. 84 1. 22 0.82 C102, percent 0.70 1.02 1. 09 H1804, percent 0. 85 0. 79 80;, percent 0. 64 1. 00

1 (JD-chlorination with chlorine dioxide added, A

1 This pulp was centri-cleaned and given 2 sulfur di suliuric acid with chlorine dioxide added, Hsodium hypochlorite, D chlorine dioxide, E-caustic extraction.

to 2.5 pH, and to F. before making viscose handsheets.

oxide treatments for 30 min. each at 5.0% consistency, 2.0

TABLE 4.HARDWOOD DISSOLVING PULPS Type pulp Mixed 100% Mixed 100% Hardwoods Sweetgum Hardwoods Sweetgum Sample number 00-245 00-248 00-247 00-246 Laboratory bleaching CDEHDED 1 CDEHDED 1 ADEHDED ADEHDED l Viscose Solution Quality Aging temperature, C 26. 2 27 27. 8 26. 2 Aging time, hr. (40 vise.) 73 73 73 78 Temperature requirement (40 visc.) 25. 6 25.8 28. 2 26. 0 Spinning viscosity (36 hr.) 35. 3 32. 6 43. 4 38. 3

Rayon Quality Denier, g 173. 6 175. 2 174. 2 173. 8 Tensile, gJdenier cond. (Aird 4.00 4. 34 4. 32 4. 47 Elongation, percent cond 9. 2 10. 2 10. 0 10. 7 Tensile, gJdenier wet- 2. 52 2. 80 2. 77 2. 84 Elongation, percent w 16. 6 18.0 17. 2 18. 4 Tensile, loop, g ldenier (A 2. 38 2. 49 2. 48 2. 51 Tensile, knot, gJdenier 2.07 2.09 2.07 2. 08 Elongation, loop, percent 5. 7 6.0 5. 8 5. 5 Elongation, knot, percent 4. 9 5. 1 4. 6 4. 4

See footnotes at end of tables.

TABLE 4.Gontinued Type pulp Mixed 100% Mixed 100% Hardwoods Sweetgum Hardwoods Sweetgum Sample number CO-245 CO%8 00-247 00-246 Laboratory bleaching C EHDED 1 0,. EHDED 1 AD EHDED 1 A EHDED 1 Physical Properties Basis weight, 1b./1,000 sq. ft 140 142 140 140 Apparent density, g./cc 0. 73 0.73 0. 73 0. 73 Brightness, percent G E 89. 4 88. 4 88. 7 87. 4 Fines, percent (100 mesh 21. 1 11. 4 21. 8 12. 0 Dirt, specks/720 sq. in--. 600 580 526 414 Chemical Analyses Viscosity, TAPPI T-230, 1% CED, cp 100 100 101 102 Alpha cellulose, percent 95. 0 95. 4 95. 2 95. 3 Ash, percent 0. 03; 0.02 0.03 0.03 Pentosans, percent. 3. 6 3. 3 3. 5 3. 2 Resins, percent 0. 076 0. 047 0. 064 0. 044 7 14% NaOH solub1l1ty 6. 03 5. 39 5. 97 5.09 Copper number 0. 32 0. 24 0. l8 0. 16

1 Cnchlorinatlon with chlorine dioxide added, lie-sulfuric acid with chlorine dioxide added, H-sodium hypochlorite, D-chlorine dioxide, E-caustic extraction.

What is claimed is:

1. in a process of preparing wood pulp by subjecting it to a sequence of bleaching and caustic extraction steps without direct chlorination, the step of bleaching the pulp first with chlorine dioxide under acid conditions produced by the addition of an acid to the pulp and then with at least one hy-pochlorite.

2. In a process of preparing cellulosic pulp by subjecting it to a sequence of bleaching and caustic extraction steps without direct chlorination, the step of bleaching the pulp first with chlorine dioxide under acid conditions produced by the addition of an acid to the pulp.

3. Dissolving grade cellulosic pulp made in accordance with claim 2.

References Cited UNITED STATES PATENTS 2,494,542 1/1950 Casciani l6278 2,516,788 7/ 1950 Day 16 2-89 2,587,064 2/ 1952 Rapson 16 2-88 3,020,196 2/ 1962 Schub'er 162-78 35 3,061,504 10/1962 Mutton 16 289 DONALL H. SYLVESTER, Primary Examiner. HOWARD R. CAINE, Examiner. 

1. IN A PROCESS OF PREPARING WOOD PULP BY SUBJECTING IT TO A SEQUENCE OF BLEACHING AND CAUSTIC EXTRACTION STEPS WITHOUT DIRECT CHLORINATION, THE STEP OF BLEACHING THE PULP FIRST WITH CHLORINE DIOXIDE UNDER ACID CONDITIONS PRODUCED BY THE ADDITION OF AN ACID TO THE PULP AND THEN WITH AT LEAST ONE HYPOCHLORITE. 